arXiv subjects

Boris Safonov

Publications and source records attributed to Boris Safonov.

At least 19 recordsLinked to original sources

arXiv2026

TOI-6884b: A low-mass brown dwarf transiting a slightly evolved star

We report the discovery of a low-mass transiting brown dwarf orbiting TOI-6884 (TIC~156514476, $T_{\rm mag}=11.4$) from NASA's \textit{Transiting Exoplanet Survey Satellite} (\textit{TESS}) mission. The \textit{TESS} light curves initially suggested an orbital period of $\sim$14.42~days; however, our high-precision ground-based radial velocity measurements and multi-epoch time-series photometry reveal this to be a harmonic alias. We determine the true orbital period to be $4.808264^{+0.000015}_{-0.000014}$~days and confirm the substellar nature of the companion. TOI-6884b has a mass of $26.32^{+0.98}_{-0.93}\,M_{\mathrm{J}}$, a radius of $0.927^{+0.51}_{-0.52}\,R_{\mathrm{J}}$, and resides on a nearly circular orbit ($e=0.067^{+0.010}_{-0.012}$). Its host star is a late F-type slightly evolved star with $M_\star = 1.410^{+0.075}_{-0.069}\,M_\odot$,\msun, $R_\star = 1.840^{+0.072}_{-0.073}\,R_\odot$, $\log{g} = 4.057^{+0.045}_{-0.039}$, $[{\rm Fe/H}] = 0.094^{+0.073}_{-0.068}$~dex, and $T_{\rm eff}=6330^{+180}_{-160}$,\mathrm{K}$. TOI-6884b is a key addition to the small population of well-characterized transiting brown dwarfs orbiting host stars that have evolved off the main sequence. The detection of such systems will contribute to our understanding of the dynamical histories and structural evolution of short-period substellar companions around evolved stars.

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arXiv2026

TOI-3862 b: A dense super-Neptune deep in the hot Neptune desert

The structure and evolution of close-in exoplanets are shaped by atmospheric loss and migration processes, which give rise to key population features such as the hot Neptune desert, ridge, and savanna - regions of the period-radius space whose boundaries offer critical insights into planetary formation and survival. As part of the KESPRINT collaboration, we selected the TESS transiting planet candidate TOI-3862.01 for radial velocity follow-up to confirm its planetary nature and characterize its mass and bulk properties. This planet candidate is of particular interest due to its position in the middle of the hot Neptune desert, making it a valuable probe for testing theories of planet migration and atmospheric loss. We confirmed the planetary nature and determined the mass of TOI-3862.01 (hereinafter TOI-3862b) by performing a joint fit with both transit and radial velocity data, precisely characterizing the bulk properties of this planet. TOI-3862b is a super-Neptune on a 1.56-day orbit around a Sun-like star with an effective temperature of 5300$\pm$50K. It has a mass of 53.7$_{-2.9}^{+2.8}$ M$_{\oplus}$ and a radius of 5.53$\pm$0.18 R$_{\oplus}$, corresponding to a density of 1.7$\pm$0.2 g/cm^3. This places it among the rare population of hot and dense super-Neptune desert planets. TOI-3862b, residing deep in the hot Neptune desert, represents a rare occurrence in an otherwise sparsely populated region, offering a valuable opportunity to probe the processes that may allow planets to survive in such environments.

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arXiv2025

Precise mass and radius determination for two new and one known Neptune-sized planets around G Dwarf hosts

As part of the KESPRINT collaboration, we present the discovery and characterization of three exoplanets in the sub-Neptune to super-Neptune regime, spanning key regions of the exo-Neptunian landscape. TOI-1472c and TOI-1648b are newly discovered sub-Neptunes, while TOI-1472b is a previously known super-Neptune for which we provide an improved mass measurement. These planets have orbital periods of 6--15 days and radii of 2.5--4.1 R$_\oplus$, probing regions where planet formation and atmospheric evolution remain poorly understood. We combine TESS transit photometry with ground-based radial velocities to determine precise masses, radii, and orbital properties. TOI-1472b has a mass of $18.0^{+0.84}_{-0.85}$ M$_\oplus$ and a radius of $4.06 \pm 0.10$ R$_\oplus$, TOI-1472c has a mass of $21.1^{+0.96}_{-0.99}$ M$_\oplus$ and a radius of $3.33 \pm 0.08$ R$_\oplus$, and TOI-1648b has a mass of $7.4^{+1.1}_{-1.3}$ M$_\oplus$ and a radius of $2.54^{+0.14}_{-0.12}$ R$_\oplus$. The planets exhibit a range of eccentricities (0.041--0.178), indicating diverse evolutionary histories. TOI-1648b, with a high Transmission Spectroscopy Metric (TSM $\sim$59), is a promising target for atmospheric characterization. Together, these three planets provide precise constraints on the structure, composition, and dynamical evolution of small to intermediate-sized exoplanets, enriching our understanding of the exo-Neptunian landscape.

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arXiv2025

Dynamical Analysis of the HD 169142 Planet-Forming Disk: Twelve Years of High-Contrast Polarimetry

We present a dynamical analysis of the HD 169142 planet-forming disk based on high-contrast polarimetric imaging over a twelve-year observational period, offering insights into its disk evolution and planet-disk interactions. This study explores the evolution of scattered-light features and their relationship with millimeter continuum emission. Archival visible-to-near-infrared scattered-light observations from NACO, SPHERE, and GPI combined with new observations from SCExAO reveal persistent non-axisymmetric structures in both the inner and outer rings of the disk. Through Keplerian image transformations and phase cross-correlation techniques, we show that the azimuthal brightness variations in the inner ring follow the local Keplerian velocity, suggesting these are intrinsic disk features rather than planet-induced spirals or shadows. The motion of the outer ring is weakly detected, requiring a longer observational baseline for further confirmation. Comparing scattered-light features with ALMA 1.3 mm-continuum data, we find that the scattered light traces the edges of dust structures in the inner ring, indicating complex interactions and a leaky dust trap around the water-ice snowline. These findings highlight the capability of long-term monitoring of circumstellar disks to distinguish planetary influences from Keplerian disk dynamics.

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arXiv2025

Resolving Pleiades binary stars with Gaia and speckle interferometric observations

The Pleiades is the most prominent open star cluster visible from Earth and an important benchmark for simple stellar populations, unified by common origin, age, and distance. Binary stars are its essential ingredient, yet their contribution remains uncertain due to heavy observational biases. A resolved multiplicity survey was conducted for a magnitude-limited G < 15mag sample of 423 potential cluster members, including sources with poorly fitted astrometric solutions in Gaia DR3. Speckle interferometric observations at the 2.5 meter telescope of SAI MSU observatory were combined with Gaia data, enabling the identification of 61 resolved binary or multiple systems within the 0.04 - 10 arcsec (5 - 1350 au) separation range. With speckle observations, we discovered 21 components in 20 systems. The existence of a Merope (23 Tau) companion is confirmed after several previous unsuccessful attempts. We show that the Gaia multipeak fraction is a strong predictor of subarcsecond multiplicity, as all sources with ipd_frac_multi_peak > 4% are successfully resolved. We found that 10% of Pleiades stars have a companion with a mass ratio q > 0.5 within projected separation of 27 < s < 1350 au, and confirm a deficit of wide binaries with s > 300 au. An observed dearth of wide pairs with large mass ratio (q > 0.55) may imprint the transition from hard to soft binaries regime at the early stages of cluster evolution. The total binary fraction for q > 0.5 systems is extrapolated to be around 25%.

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arXiv2024

Visible-Light High-Contrast Imaging and Polarimetry with SCExAO/VAMPIRES

We present significant upgrades to the VAMPIRES instrument, a visible-light (600 nm to 800 nm) high-contrast imaging polarimeter integrated within SCExAO on the Subaru telescope. Key enhancements include new qCMOS detectors, coronagraphs, polarization optics, and a multiband imaging mode, improving sensitivity, resolution, and efficiency. These upgrades position VAMPIRES as a powerful tool for studying sub-stellar companions, accreting protoplanets, circumstellar disks, stellar jets, stellar mass-loss shells, and solar system objects. The instrument achieves angular resolutions from 17 mas to 21 mas and Strehl ratios up to 60\%, with 5$σ$ contrast limits of $10^{\text{-}4}$ at 0.1'' to $10^{\text{-}6}$ beyond 0.5''. We demonstrate these capabilities through spectro-polarimetric coronagraphic imaging of the HD 169142 circumstellar disk, ADI+SDI imaging of the sub-stellar companion HD 1160B, narrowband H$α$ imaging of the R Aqr emission nebula, and spectro-polarimetric imaging of Neptune.

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arXiv2024

TOI-1173 A $b$: The First Inflated Super-Neptune in a Wide Binary System

Among Neptunian mass exoplanets ($20-50$ M$_\oplus$), puffy hot Neptunes are extremely rare, and their unique combination of low mass and extended radii implies very low density ($ρ< 0.3$~g~cm$^{-3}$). Over the last decade, only a few puffy planets have been detected and precisely characterized with both transit and radial velocity observations, most notably including WASP-107~$b$, TOI-1420~$b$, and WASP-193 $b$. In this paper, we report the discovery of TOI-1173 A $b$, a low-density ($ρ= 0.195_{-0.017}^{+0.018}$~g~cm$^{-3}$) super-Neptune with $P = 7.06$ days in a nearly circular orbit around the primary G-dwarf star in the wide binary system TOI-1173 A/B. Using radial velocity observations with the MAROON-X and HIRES spectrographs and transit photometry from TESS, we determined a planet mass of $M_{\rm{p}} = 27.4\pm1.7\ M_{\oplus}$ and radius of $R_{\rm{p}} = 9.19\pm0.18\ R_{\oplus}$. TOI-1173 A $b$ is the first puffy Super-Neptune planet detected in a wide binary system (projected separation $\sim 11,400$~AU). We explored several mechanisms to understand the puffy nature of TOI-1173 A $b$, and showed that tidal heating is the most promising explanation. Furthermore, we demonstrate that TOI-1173 A $b$ likely has maintained its orbital stability over time and may have undergone von-Zeipel-Lidov-Kozai migration followed by tidal circularization given its present-day architecture, with important implications for planet migration theory and induced engulfment into the host star. Further investigation of the atmosphere of TOI-1173 A $b$ will shed light on the origin of close-in low-density Neptunian planets in field and binary systems, while spin-orbit analyses may elucidate the dynamical evolution of the system.

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arXiv2024

The TESS-Keck Survey XX: 15 New TESS Planets and a Uniform RV Analysis of all Survey Targets

The Transiting Exoplanet Survey Satellite (TESS) has discovered hundreds of new worlds, with TESS planet candidates now outnumbering the total number of confirmed planets from $\textit{Kepler}$. Owing to differences in survey design, TESS continues to provide planets that are better suited for subsequent follow-up studies, including mass measurement through radial velocity (RV) observations, compared to Kepler targets. In this work, we present the TESS-Keck Survey's (TKS) Mass Catalog: a uniform analysis of all TKS RV survey data which has resulted in mass constraints for 126 planets and candidate signals. This includes 58 mass measurements that have reached $\geq5σ$ precision. We confirm or validate 32 new planets from the TESS mission either by significant mass measurement (15) or statistical validation (17), and we find no evidence of likely false positives among our entire sample. This work also serves as a data release for all previously unpublished TKS survey data, including 9,204 RV measurements and associated activity indicators over our three year survey. We took the opportunity to assess the performance of our survey, and found that we achieved many of our goals including measuring the mass of 38 small ($<4R_{\oplus}$) planets, nearly achieving the TESS mission's basic science requirement. In addition, we evaluated the performance of the Automated Planet Finder (APF) as survey support and observed meaningful constraints on system parameters due to its more uniform phase coverage. Finally, we compared our measured masses to those predicted by commonly used mass-radius relations and investigated evidence of systematic bias.

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arXiv2023

TESS Spots a Super-Puff: The Remarkably Low Density of TOI-1420b

We present the discovery of TOI-1420b, an exceptionally low-density ($ρ= 0.08\pm0.02$ g cm$^{-3}$) transiting planet in a $P = 6.96$ day orbit around a late G dwarf star. Using transit observations from TESS, LCOGT, OPM, Whitin, Wendelstein, OAUV, Ca l'Ou, and KeplerCam along with radial velocity observations from HARPS-N and NEID, we find that the planet has a radius of $R_p$ = 11.9 $\pm$ 0.3 $R_\Earth$ and a mass of $M_p$ = 25.1 $\pm$ 3.8 $M_\Earth$. TOI-1420b is the largest-known planet with a mass less than $50M_\Earth$, indicating that it contains a sizeable envelope of hydrogen and helium. We determine TOI-1420b's envelope mass fraction to be $f_{env} = 82^{+7}_{-6}\%$, suggesting that runaway gas accretion occurred when its core was at most $4-5\times$ the mass of the Earth. TOI-1420b is similar to the planet WASP-107b in mass, radius, density, and orbital period, so a comparison of these two systems may help reveal the origins of close-in low-density planets. With an atmospheric scale height of 1950 km, a transmission spectroscopy metric of 580, and a predicted Rossiter-McLaughlin amplitude of about $17$ m s$^{-1}$, TOI-1420b is an excellent target for future atmospheric and dynamical characterization.

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arXiv2023

Another Shipment of Six Short-Period Giant Planets from TESS

We present the discovery and characterization of six short-period, transiting giant planets from NASA's Transiting Exoplanet Survey Satellite (TESS) -- TOI-1811 (TIC 376524552), TOI-2025 (TIC 394050135), TOI-2145 (TIC 88992642), TOI-2152 (TIC 395393265), TOI-2154 (TIC 428787891), & TOI-2497 (TIC 97568467). All six planets orbit bright host stars (8.9 <G< 11.8, 7.7 <K< 10.1). Using a combination of time-series photometric and spectroscopic follow-up observations from the TESS Follow-up Observing Program (TFOP) Working Group, we have determined that the planets are Jovian-sized (R$_{P}$ = 1.00-1.45 R$_{J}$), have masses ranging from 0.92 to 5.35 M$_{J}$, and orbit F, G, and K stars (4753 $<$ T$_{eff}$ $<$ 7360 K). We detect a significant orbital eccentricity for the three longest-period systems in our sample: TOI-2025 b (P = 8.872 days, $e$ = $0.220\pm0.053$), TOI-2145 b (P = 10.261 days, $e$ = $0.182^{+0.039}_{-0.049}$), and TOI-2497 b (P = 10.656 days, $e$ = $0.196^{+0.059}_{-0.053}$). TOI-2145 b and TOI-2497 b both orbit subgiant host stars (3.8 $<$ $\log$ g $<$4.0), but these planets show no sign of inflation despite very high levels of irradiation. The lack of inflation may be explained by the high mass of the planets; $5.35^{+0.32}_{-0.35}$ M$_{\rm J}$ (TOI-2145 b) and $5.21\pm0.52$ M$_{\rm J}$ (TOI-2497 b). These six new discoveries contribute to the larger community effort to use {\it TESS} to create a magnitude-complete, self-consistent sample of giant planets with well-determined parameters for future detailed studies.

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arXiv2023

TOI-1695 b: A Water World Orbiting an Early M Dwarf in the Planet Radius Valley

Characterizing the bulk compositions of transiting exoplanets within the M dwarf radius valley offers a unique means to establish whether the radius valley emerges from an atmospheric mass loss process or is imprinted by planet formation itself. We present the confirmation of such a planet orbiting an early M dwarf ($T_{\rm mag} = 11.0294 \pm 0.0074, M_s = 0.513 \pm 0.012\ M_\odot, R_s = 0.515 \pm 0.015\ R_\odot, T_{\rm eff} =3690\pm 50 K$): TOI-1695 b ($P = 3.13$ days, $R_p = 1.90^{+0.16}_{-0.14}\ R_\oplus$). TOI-1695 b's radius and orbital period situate the planet between model predictions from thermally-driven mass loss versus gas depleted formation, offering an important test case for radius valley emergence models around early M dwarfs. We confirm the planetary nature of TOI-1695 b based on five sectors of TESS data and a suite of follow-up observations including 49 precise radial velocity measurements taken with the HARPS-N spectrograph. We measure a planetary mass of $6.36 \pm 1.00\ M_\oplus$, which reveals that TOI-1695 b is inconsistent with a purely terrestrial composition of iron and magnesium silicate, and instead is likely a water-rich planet. Our finding that TOI-1695 b is not terrestrial is inconsistent with the planetary system being sculpted by thermally driven mass loss. We present a statistical analysis of seven well-characterized planets within the M dwarf radius valley demonstrating that a thermally-driven mass loss scenario is unlikely to explain this population.

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arXiv2022

The TESS-Keck Survey. XI. Mass Measurements for Four Transiting sub-Neptunes orbiting K dwarf TOI-1246

Multi-planet systems are valuable arenas for investigating exoplanet architectures and comparing planetary siblings. TOI-1246 is one such system, with a moderately bright K dwarf ($\rm{V=11.6,~K=9.9}$) and four transiting sub-Neptunes identified by TESS with orbital periods of $4.31~\rm{d},~5.90~\rm{d},~18.66~\rm{d}$, and $~37.92~\rm{d}$. We collected 130 radial velocity observations with Keck/HIRES and TNG/HARPS-N to measure planet masses. We refit the 14 sectors of TESS photometry to refine planet radii ($\rm{2.97 \pm 0.06~R_\oplus},\rm{2.47 \pm 0.08~R_\oplus}, \rm{3.46 \pm 0.09~R_\oplus}$, $\rm{3.72 \pm 0.16~R_\oplus}$), and confirm the four planets. We find that TOI-1246 e is substantially more massive than the three inner planets ($\rm{8.1 \pm 1.1 M_\oplus}$, $\rm{8.8 \pm 1.2 M_\oplus}$, $\rm{5.3 \pm 1.7 M_\oplus}$, $\rm{14.8 \pm 2.3 M_\oplus}$). The two outer planets, TOI-1246 d and TOI-1246 e, lie near to the 2:1 resonance ($\rm{P_{e}/P_{d}=2.03}$) and exhibit transit timing variations. TOI-1246 is one of the brightest four-planet systems, making it amenable for continued observations. It is one of only six systems with measured masses and radii for all four transiting planets. The planet densities range from $\rm{0.70 \pm 0.24}$ to $3.21 \pm 0.44 \rm{g/cm^3}$, implying a range of bulk and atmospheric compositions. We also report a fifth planet candidate found in the RV data with a minimum mass of 25.6 $\pm$ 3.6 $\rm{M_\oplus}$. This planet candidate is exterior to TOI-1246 e with a candidate period of 93.8 d, and we discuss the implications if it is confirmed to be planetary in nature.

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arXiv2021

Apparent motion of the circumstellar envelope of CQ Tau in scattered light

The study of spiral structures in protoplanetary disks is of great importance for understanding of processes in the disks, including planet formation. Bright spiral arms were detected in the disk of young star CQ Tau by Uyama et al. (2020) in H and L bands. The spiral arms are located inside the gap in millimeter size dust, recovered earlier using ALMA observations (Ubeira Gabellini et al. 2019). To explain the gap, Ubeira Gabellini et al. (2019) proposed the existence of a planet with the semimajor axis of 20 AU. We obtained multiepoch observations of a spiral feature in the circumstellar envelope of CQ Tau in Ic band using a novel technique of differential speckle polarimetry. The observations covering period from 2015 to 2021 allow us to estimate the pattern speed of spiral: $-0.2\pm1.1^{\circ}$/yr (68% credible interval, positive value indicates counter-clockwise rotation), assuming face-on orientation of the disk. This speed is significantly smaller than expected for a companion-induced spiral, if the perturbing body has the semimajor axis of 20 AU. We emphasize that the morphology of the spiral structure is likely to be strongly affected by shadows of a misaligned inner disk detected by Eisner et al. (2004).

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arXiv2021

Useful relations for the analysis of stellar scintillation at the entrance pupil of a telescope

The development of new techniques for characterizing atmospheric optical turbulence (OT) has become an active topic of research again in recent years. In order to facilitate these studies, we reconsidered known theoretical results and obtained some new practically useful conclusions. We introduce a dimensionless Fresnel filter, which allows us to approximate a polychromatic weighting function (WF) by a monochromatic one with a typical precision of several percent. A so-called dimensionless WF can be easily scaled for a receiving aperture of any size. For the case of a circular aperture and monochromatic radiation, an analytical expression for the WF was found. The WFs for a square aperture and for a circular aperture match with relative difference less than 0.01 if the circular aperture diameter is 1.15 times larger than the square aperture side. A linear digital filter can be applied to the scintillation signal from an image detector. As an example of digital filtering, we considered the power law filter $\propto f^{5/3}$ with the WF being constant in a wide range of altitudes. We discuss the main limitations of this approach for measuring OT integral: finite pixel size, aliasing, and finite image detector size.

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arXiv2020

Caucasian mountain observatory of Sternberg astronomical institute: six years of operation

The new SAI MSU observatory 2.5-meter telescope and capabilities of its current instrumentation are described. The facility operates actively since 2014 in parallel to the engineering works. It has delivered a number of prominent results in the field of optical and near-infrared photometry and spectroscopy as well as newly developed observational techniques like differential speckle polarimetry.

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arXiv2020

Differential Speckle Polarimetry of Betelgeuse in 2019-2020: the rise is different from the fall

Recently published episodic spectral (Levesque and Massey, 2020) and high angular resolution (Montarges et al, 2020) observations of Betelgeuse suggest that the deep minimum of 2019-2020 was caused by an enhanced dust abundance in the stellar atmosphere. Detailed monitoring of such events may prove useful for constructing consistent physical models of mass loss by evolved stars. For such observations it is fundamentally important to employ methods resolving an inhomogeneous stellar atmosphere. We present the differential speckle polarimetric observations of Betelgeuse at 2.5-m telescope of Caucasian Mountain Observatory of SAI MSU covering the period of 2019-2020 minimum. The observations were secured on 17 dates at wavelengths 465, 550, 625 and 880 nm. The circumstellar reflection nebula with the angular size of $\approx0.1^{\prime\prime}$ was detected for all the dates and at all wavelengths. The morphology of the nebula changed significantly over the observational period. Net polarized brightness of the envelope remained constant until February 2020, while the stellar $V$ band flux decreased 2.5 times. Starting from mid-February 2020, polarized flux of the envelope rose 2.1 times, at the same time the star returned to the pre-minimum state of October 2019. Basing on these data and our low resolution spectrum obtained on 2020-04-06 we confirm a conclusion that the minimum is caused by the formation of a dust cloud located on the line of sight. A quantitative characterisation of this cloud will be possible when the data on its thermal radiation are employed.

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arXiv2019

The envelope of the semiregular variable V CVn

V CVn is a semiregular variable star with a V-band amplitude of $\approx2$ mag. This star has an unusually high amplitude of polarimetric variability: up to 6 per cent. It also exhibits a prominent inverse correlation between the flux and the fraction of polarization and a substantial constancy of the angle of polarization. To figure out the nature of these features, we observed the object using the Differential Speckle Polarimetry at three bands centered on 550, 625 and 880 nm using the 2.5-m telescope of Sternberg Astronomical Institute. The observations were conducted on 20 dates distributed over three cycles of pulsation. We detected an asymmetric reflection nebula consisting of three regions and surrounding the star at the typical distance of 35 mas. The components of the nebula change their brightness with the same period as the star, but with significant and different phase shifts. We discuss several hypotheses that could explain this behavior.

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arXiv2018

Differential speckle polarimetry at Cassegrain and Nasmyth foci

Polarimetric interferometry is a method allowing the study of the distribution of polarized flux at diffraction-limited resolution. Its basic observable is the ratio $\mathcal{R}$ of the visibilities of the object in two orthogonal polarizations. Here, we demonstrate how this observables can be measured with the SPeckle Polarimeter (SPP) of the 2.5-m telescope. The SPP is a combination of a dual-beam polarimeter and an EMCCD-based visible-range speckle interferometer. We propose a simple method for the correction of $\mathcal{R}$ for the instrumental polarization and polarization differential aberrations of the telescope. The polarized intensity image can be estimated from $\mathcal{R}$ under the assumption that the object is a point-like unpolarized source plus a faint extended polarized envelope. The phase of $\mathcal{R}$ can be used for measurement of the polaroastrometric signal - the difference between the photocentres of orthogonally polarized images of the object. We investigate both possibilities using observations of unpolarized stars and stars with a significant polarized circumstellar environment - $μ$~Cep and RY~Tau.

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